1 Renal Function and Fluid Balance Practice Questions

Urine formation and its route

Urine forms in the kidneys, travels through the ureters to the bladder for storage, and leaves the body through the urethra.

The kidney’s nephrons filter blood and then modify the filtered fluid. The Glomerulus filters fluid and small solutes from blood. The Renal tubule returns needed water and substances to the blood and adds certain wastes to the tubular fluid; the fluid that remains becomes urine.

Other kidney functions

In addition to making urine, the kidneys help regulate water, salts, and minerals and remove acid. They also make hormones involved in blood pressure regulation, red blood cell production, and bone health.

Interpreting urine measurements

Urine output and specific gravity describe different things. Urine output is an amount produced over time, while Specific gravity gives a rough estimate of urine concentration. Neither measurement alone provides a complete assessment.

For example, an adult weighing $70\,\mathrm{kg}$ who produces $35\,\mathrm{mL}$ of urine in $1\,\mathrm{hour}$ has an hourly output of:

Urine output is a useful observation, but it should be considered alongside the person’s overall condition and other findings.

A specific gravity of $1.028$ suggests relatively concentrated urine and is toward the concentrated end of commonly cited reference ranges. Reference ranges vary by laboratory. The result by itself does not prove dehydration, establish hydration status, diagnose normal or abnormal kidney function, or directly measure filtration rate.

Fluid compartments and body water

Total body water is divided mainly between Intracellular fluid (ICF), inside cells, and Extracellular fluid (ECF), outside cells. About two-thirds of total body water is ICF and about one-third is ECF. The two major subdivisions of ECF are interstitial fluid and intravascular fluid (plasma).

For a $70\,\mathrm{kg}$ adult male whose estimated total body water is $60\%$ of body weight, the estimates are:

• Total body water: $70\,\mathrm{kg}\times 0.60 = 42\,\mathrm{L}$.

• ICF: two-thirds of $42\,\mathrm{L}$, or about $28\,\mathrm{L}$.

• ECF: one-third of $42\,\mathrm{L}$, or about $14\,\mathrm{L}$.

These are estimates. Body-water percentage varies with factors such as body composition, age, and sex.

Fluid balance over time

Steady-state fluid balance means that, over time, total water entering the body is balanced by water leaving it. Intake and losses do not have to follow fixed amounts or routes every day. The kidneys contribute to fluid regulation by adjusting urine water and solute excretion.

To calculate a day’s net fluid balance, subtract total output from total intake. If intake is $2{,}400\,\mathrm{mL}$, urine loss is $1{,}500\,\mathrm{mL}$, and losses through other routes are $900\,\mathrm{mL}$, then:

Intake and output balance for that day. A steady-state balance does not mean that intake must always be exactly $2{,}500\,\mathrm{mL}$ per day or that the kidneys make the same urine volume every day.